Active antenna supply ?


There is a big chapter in the design document of the Kiwi about an active antenna bias tee. I cannot see anything in the schematic about this feature. It has been removed ?



  • The active antenna and bias-tee is a separate project. There was never a bias-tee on the Kiwi board.

  • You may find this interesting

  • I would think that those 1N4148 would clip with high levels present.
  • edited May 2017
    Hi James,

    The loop is relatively small so the level of field strength required for the diodes to generate IMD product above the noise floor would generally need to be quite high.

    Unless you are living very close to a Broadcast site it's unlikely that you would have problems. 

    If the loop and amplifier is subject to that sort of level of field strength it's likely that the amplifier (or radio connected to it) would already have started to generate significant of IMD products.

    I've been testing various Active antennas on the bench and I'd say that in 99% of all cases adding protection diodes to the amplifier input doesn't add any significant increase in IMD products.

    I've built various active E-Probe and Loop antennas (including the Wellbrook copy mentioned earlier in this thread), the best performers for me (so far) have been the
    LZ1AQ loop amplifier connected to a 1m diameter by 0.3m wide flat strip loop, and the Clifton Labs / Trask E-Probe designs.

    The Wellbrook design is tricky to optimise as the transistor biasing wanders around quite a lot and the coupling transformers are less than ideal. The LZ1AQ generally performs just as well and is very easy to build.

    I ommitted the input filter network, 10v regulator and used a conventional coax feed with a bias Tee. Which simplifies the design still further.

    A lot of my other experiments, tests and findings are listed on my Active Antennas web page

    If you want to compare some of the antennas I mentioned they can be selected using the Ant_Switch extension on my KiWI SDR.

    The LZ1AQ 'Fat' loop is using my 1m diameter x 0.3m flat strip loop and the other LZ1AQ loops are using 1m diameter x 14mm od LDF4-50 semi rigid coax. The 'Fat' loop is about 10dB more sensitive than the other 'Thin' loops although the amplifiers are identical.


    Martin - G8JNJ

  • I live very close to multiple MW stations thus my sensitivity on this kind of thing
  • Hi James,

    OK understood. 

    In that situation I think input protection diodes would probably be the least of your problems :-(


    Martin - G8JNJ

  • I find if the antenna's preamp does OK, I can filter the MW with notches. A "crushed" antenna devices, no hope. The Pixel Loop does fine.
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